When shifting down or when releasing the accelerator abruptly on a slippery roadway, the driven wheels of a vehicle may slip due to the braking action of the engine. In order to continue to ensure sufficient directional stability of the vehicle, the EDC raises the engine torque by giving gas lightly as soon as one driven wheel has dropped below a predefined slip threshold. The braking of the wheels is thereby reduced to an extent necessary for the directional stability.
Conventional drag-torque controls function reliably when driving on straight roads, but allow too much wheel slip in curves. Especially when cornering on a slippery roadway, this wheel slip results in a further reduction of the cornering stability. Particularly for vehicles with rear-wheel drive, the lack of cornering stability can lead to critical driving situations such as, for example, the breakaway of the vehicle tail.
FIG. 1 shows the speed of one driven wheel 1 in comparison to that of the vehicle (curve 3). If a driven wheel goes into slippage because of shifting down or upon releasing the accelerator abruptly, then its speed 1 diminishes in relation to vehicle speed 3. This can be seen in the profile of characteristic curve 1 by several speed drops 4.
The EDC control begins when wheel speed 1 has dropped below a predefined slip threshold 2. In the following, the EDC ascertains an engine torque which accelerates the powered axle of the vehicle essentially to a reference speed (usually the vehicle speed) and increases the engine torque by this value. Therefore, the engine drag-torque control causes a reduction of the drag slip, and the driven wheels are able to grip again.
During straight-ahead driving, the augmenting torque determined by known engine drag-torque controls usually suffices to ensure the vehicle stability. However, when cornering on a slippery roadway (low coefficient of friction), the magnitude and duration of the slippage is clearly too great to achieve the necessary cornering stability of the vehicle. Above all when the driver releases the accelerator while cornering on a roadway with low coefficient of friction and the driven wheels go only slowly into a state of slippage, conventional EDCs determine an augmenting torque which is much too low. The danger thereby exists that the vehicle will go out of its lane.
Therefore, an object of an present invention is to optimize an engine drag-torque control for cornering on a slippery roadway, and to improve the vehicle stability under these conditions.